In the prior art are known a great number of arrangements for automatic safety shut-off of the current supply to an electric appliance, particularly an electric range or the like. In these embodiments, the shut-off of the current supply is arranged to take place by means of timer-controlled switch, wherein the desired operating time is manually preset on the control switch timer. The switch is tripped at the lapse of the preset operating time, whereby the mains is disconnected from the appliance if the current supply has not been disconnected already prior to this instant from the user control of the electric appliance.
A disadvantage of manually presettable safety timer embodiments is that the desired duration of appliance operation time must be preset into the timer every time the electric appliance is to be switched on. Furthermore, the timer must be manually reset after the use of the appliance is terminated. Resultingly, the use of a safety timer is awkward and the user may not necessarily care to set the timer every time the appliance is operated. Due to these reasons, the benefit of a timer remains minimal.
In the prior art are also known automatic timer embodiments for shutting off current supply to an electric appliance such as an electric range or the like. A disadvantage of these automatic timers is that they provide only a fixed presentable operating time of the electric appliance during which no changes will be taken into account due to loads which vary over a period of time or are unbalanced between the phase legs.
To overcome the disadvantages of the prior-art manual and automatic timer embodiments described above, patent publication FI 100623 B teaches an automatic timer wherein the operating time of an electric appliance such an electric range or the like is determined by magnitude of the load switched on. This kind of automatic timer, however, is handicapped by being usable only in conjunction with appliances connected between one phase and the neutral of the mains or, respectively, in a wye connection to a three-phase power system.